1. Field of the Invention
The present invention relates to an apparatus for converting a wide screen television signal to a normal screen television signal. The "normal screen television signal" is defined here as a television signal which produces a television picture of a normal aspect ratio (for example, the existing 4:3 aspect ratio) displayed on a screen having such a normal aspect ratio, such as the existing NTSC, PAL and SECAM signals. The "wide screen television signal" is defined here as a television signal which produces a television picture of a wider aspect ratio than the normal aspect ratio, such a television picture being displayed on a screen having a wider aspect ratio than the normal aspect ratio, such as the recently developed HDTV signal.
2. Description of the Prior Art
Recently, various types of HDTV (High Definition Television) systems have been developed to provide a higher quality television picture than that of the existing television systems. The HDTV system is designed to use a television screen having a wider aspect ratio than the existing 4:3 aspect ratio. In other words, the HDTV television signals are the above-mentioned wide screen television signals. On the other hand, since there is widespread consumer use of television sets which are designed for the existing television systems, there is a substantial demand for a system which can convert a wide screen television signal to a normal screen television signal so that the wide screen television signal can be utilized by an existing television set.
To answer this demand, a system for converting the MUSE signal to the NTSC signal has been disclosed in Television Technology, published in October 1989 in Japan. This system is schematically shown in FIGS. 1(A) and 1(B). In the MUSE system, the aspect ratio is 16:9, the number of scanning lines is 1125, and the field frequency is 60 Hz. Referring to FIG. 1(A), the MUSE signal having the 16:9 aspect ratio is converted to the NTSC signal having the 4:3 aspect ratio, or more specifically, a part of the MUSE picture is extracted so that the aspect ratio of the extracted part becomes 4:3. Three parts to be extracted are predetermined: a left part L, the center part C and the right part R. One of the three parts can be selected by a viewer according to the viewer's preference. FIG. 1(B) shows a main part of the system in which the MUSE-to-NTSC conversion is made by a random access memory (RAM) 1 which is controlled by write and read clocks having different frequencies from each other. A write address counter 2 is responsive to the write clock (16.2 MHz) for writing the MUSE signal into the RAM 1. A read address counter 3 is responsive to the read clock (5.04 MHz) for reading out the signal stored in the RAM 1 to obtain a time-axis converted signal, which is the NTSC signal. The write clock is applied to the RAM 1 through a gate 4 which is controlled by a window signal so that the MUSE signal is written into the RAM 1 only at the portion corresponding to the 4:3 aspect ratio. The timing of the active period of the window signal is preset by, for example, using a manual selection switch operated by a viewer according to the viewer's preference. That is, when the viewer wishes to watch the left part L of the MUSE picture the viewer may select the left part L by operating the manual selection switch.
In the above system, however, the selection of the extracting area is made by a viewer. The viewer may find troublesome the operation of the manual selection switch even if the viewer wishes to change the part to be displayed on the NTSC television screen.